[users@httpd] mod_cache behavior with proxied images - urls not cached

[edward.dunkle@xxxxxxxxxxx]

We are looking into caching images on
Apache that are proxied from a backend server.  However, it looks
like a lot of traffic will still go to the backend.

We have Apache configured like this:

<IfModule mod_cache.c>

    <IfModule mod_mem_cache.c>
        CacheIgnoreCacheControl
On 
        CacheIgnoreNoLastMod
On
        CacheIgnoreHeaders
Set-Cookie 
        CacheDefaultExpire
3600
        CacheMaxExpire
86400
        CacheEnable
mem /backend/images 
        MCacheRemovalAlgorithm
LRU 
        MCacheSize
32768
        MCacheMaxObjectCount
1024
        MCacheMinObjectSize
1
        MCacheMaxObjectSize
1048576
    </IfModule>

</IfModule> 

If I remove all my local temporary files,
then this seems to work well.  The image is retrieved from the backend
and cached on Apache.  Then subsequent requests are handled by Apache.

But, we have a large base of existing
clients who will most likely already have local copies of images.  When
such a client loads a page, the requests are passed through to the backend
only to find that they have not been modified and a 302 is returned for
each image.  This causes a lot of unnecessary traffic to the backend.
 How can we avoid that?

When Apache starts up, the cache is
empty.  These images are not getting cached because the client issues
a conditional GET.  The file is not in the Apache cache, so the request
is passed to the backed.
Then the backend responds with a 302
so Apache is not able to retrieve the image and put it in the cache.  Do
we have to initialize the cache somehow?  

Thanks,
Ed